The Apostle himself in Rom. 6 & 7 shows quite openly that concupiscence is sin properly so called: for not only does he often call it sin, but not good, evil, to be held in hatred, fighting against the law of God, to be crucified, mortified, and destroyed, and rendering that man miserable in whom it resides.
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Moral corruption fighting against the law, for the crucifying and mortifying of which Christ himself died on the cross, if it be not properly sin, then Scripture has not explained to us what the property of any habitual sin is.
Whatever differs from the norm of the law is sin: but concupiscence in the regenerate differs from the norm of the law. Therefore.
This very thing cannot be conceived, how something by way of an internal habit could be the cause of sin and not be sin. An internal cause inclining to act well is good.
This corruption of nature is here forbidden, for it is truly and properly sin, Rom. 6:12 and 7:17. It is the flesh that lusteth against the Spirit, Gal. 5:18; and if sin, it must be contrary to and forbidden by the law.